How can I cast artificially darker shadows?
You have three options.


You could add a light that is only visible to the shadow path, so it won't add any light, but it will cast shadows.  If you're happy with the scene otherwise, you should duplicate one of the existing lights and move it forward or back just a touch.
Another option is to add a "negative light", where you add a light with a color of white, and a negative value for the strength.  There aren't such things in real life (except maybe black-holes) but for some lighting effects it is a neat trick to add darkness.
Lastly you could do something with render layers, and render passes to get the shadow on it's own render layer and then control how dark it is during composting.